Commands:

last -10   :shows last 10 successful logins
lastb -5   :shows last 5 failed login attempts

df -h         :disk
dd if=/dev/zeros of=/dev/sd? bs=1M   :zero a disk
free          :free memory

cp         :copy
ls -l
rm file    :delete file
shutdown -h now   :  -r for reboot

uname -a   :show name, linux info etc
w          :shows logged in users

pgrep firefox    :gets the PID of process (firefox in this case)
kill PID         :kills the process

top      :also may want to install atop and htop

:To remove old linux kernels
dpkg --list | grep linux-image          :shows installed images
sudo apt-get purge linux-image-x.x.x-xx-generic   :removes it

:Updates might fail if run out of inodes
df -i      :shows inode usage
:Likely that too many linux-headers exist, such as /usr/src/linux-headers-#.#.#-#
sudo rm -rf linux-headers-#.#.#-#

ifconfig
arp-scan --interface=wlan0 --localnet  : or eth0

ssh username@ip.address {-l root}

scp username@ip.address:filename filename :vice versa

nslookup {ip address}

If server set up with openssh-server, it will get attacked, mostly from china. 
To block brute-force attacks, install iptables-persistent, then use it to save rules:
iptables -A INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent --set 
iptables -A INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent --update --seconds 60 --hitcount 4 -j DROP